CMOS image sensors are increasingly being used as low cost imaging devices. A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells includes a photogate, photoconductor, or photodiode having an associated charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a sensing node, and a transistor for resetting the sensing node to a predetermined charge level prior to charge transference. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the sensing node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the sensing node accompanied by charge amplification; (4) resetting the sensing node to a known state; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the sensing node.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe the operation of conventional CMOS image sensors and are assigned to Micron Technology, Inc., the contents of which are incorporated herein by reference.
A schematic diagram of a conventional CMOS four-transistor (4T) pixel cell 10 is shown in FIG. 1. The CMOS pixel cell 10 generally comprises a photosensor 14 for generating and collecting charge generated by light incident on the pixel cell 10, and a transfer transistor 17 for transferring photoelectric charges from the photosensor 14 to a sensing node, typically a floating diffusion region 5. The floating diffusion region 5 is electrically connected to the gate of an output source follower transistor 19. The pixel cell 10 also includes a reset transistor 16 for resetting the floating diffusion region 5 to a predetermined voltage Vaa-pix; and a row select transistor 18 for outputting a signal Vout from the source follower transistor 19 to an output terminal in response to an address signal.
FIG. 2 is a cross-sectional view of a portion of the pixel cell 10 of FIG. 1 showing the photo-conversion device 14, transfer transistor 17 and reset transistor 16. The exemplary CMOS pixel cell 10 has a photosensor 14 that may be formed as a pinned photodiode. The photodiode photosensor 14 has a p-n-p construction comprising a p-type surface layer 13 and an n-type photodiode region 12 within a p-type active layer 11. The transfer transistor 17 and reset transistor 16 gates sit on a thin gate oxide layer 15. The photodiode photosensor 14 is adjacent to and partially underneath the transfer transistor 17. The reset transistor 16 is on a side of the transfer transistor 17 opposite the photodiode photosensor 14. As shown in FIG. 2, the reset transistor 16 includes a source/drain region 2. The floating diffusion region 5 is between the transfer and reset transistors 17, 16.
In the CMOS pixel cell 10 depicted in FIGS. 1 and 2, electrons are generated by light incident on the photodiode photosensor 14 and are stored in the n-type photodiode region 12. These charges are transferred to the floating diffusion region 5 by the transfer transistor 17 when the transfer transistor 17 is activated. The source follower transistor 19 produces an output signal based on the transferred charges. A maximum output signal is proportional to the number of electrons extracted from the n-type photodiode region 12.
One common problem associated with conventional imager pixel cells, such as pixel cell 10, is dark current. Dark current is current generated as a photosensor signal in the absence of light. Dark current may be caused by many different factors, including, but not limited to: contaminants that diffuse into the photosensor silicon during the gate-formation steps of pixel fabrication; photosensor junction leakage, i.e., diffusion of ions across the p-n-p layers of the photosensor; and photon reflection at the photosensor surface. Dark current is detrimental to the operation and performance of a photosensor. Accordingly, it is desirable to provide an isolation technique that prevents dark current by providing a contaminant barrier to protect the photosensor that can also contain the underlying ion distribution to maintain the photosensor junction and reduce photon reflection at the photosensor surface.